1. Field of the Invention
This invention relates to devices for coupling rotating shafts together, and in particular, to a flexible shaft coupler device that compensates for various misalignments between drive and driven shafts.
2. Description of Related Art
Numerous flexible or elastic shaft couplings are in use for coupling drive and driven shafts in systems having rotating mechanical parts. A motor, for example, may have to be coupled to a gear box and for this application it is far superior in terms of compactness and economics to use a flexible shaft coupling rather than establish precise parallel alignment between the two shafts. In fact, many rotary systems have bearings and other support elements which are designed to accommodate movement about the axis of a shaft upon start-up or during operation. In addition, thermal expansion of components, system vibration, and other factors introduce misalignments that can best be taken up by flexible shaft couplings.
Flexible shaft couplings are also used where no substantial torsional load may be involved, but when one member, such as a rotary transducer or encoder, may be coupled to another member, such as a servo motor, to provide a precise indication of motor angular position. Not only must shaft misalignment be compensated for, but the amount of rotational displacement or "wind-up" must be essentially eliminated as a material error.
Shaft misalignments can take any one or all of several forms. There can be parallel misalignment, with the rotational axes of the shafts being parallel but not coaxial, and also angular misalignment, where the rotational axis of the shafts intersect (are not parallel). Angular misalignment can be in more than one plane. Most applications contain a combination of both parallel and angular misaligmuents, often referred to as skewed misalignment. In addition, as noted, the axial displacements between the shafts may also vary, statically or dynamically for different reasons.
Consequently, as the two coupled but somewhat misaligned shafts rotate, varying loads are exerted in cyclic fashion on the flexible shaft coupling. During each rotation, any given part of the coupling is subject to compression through one limited arc and later subject to expansion through another limited arc. These variations continually exist during rotation, and consequently, strength, modulus, wear and fatigue factors all must be accounted for in choosing the materials and dimensions for the elastic coupling.
The principal existing approaches, those identified here being by no means exclusive, to the design of elastic shaft couplings are based on the use of flexible diaphragms, transverse to the axes of rotation, flexible bellows concentric with the axes of rotation, and flexible sleeves concentric with the axes of rotation. The sleeves are made flexible by being divided into columns which can yield somewhat under the deflections involved. All three designs permit axial and torsional variations to be accommodated, but all three have specific limitations that relate not only to their performance, but the suitability of their configuration for use in practical installations.
The flexible shaft element must be coupled to the shafts, as by hubs with set screws, keyways, clamps, splines, tapered members, bonding or other means. The radial dimension of the coupling can usually be a substantial portion of the radial dimension of a drive or driven member, which is generally larger. Length, however, is often the most important consideration since it is generally desired to have the drive and driven elements close together.
The yieldable portion of the coupler must observe certain constraints, depending upon the design, and also must be configured so as to be compactly attachable by a hub or other means to the associated shaft portion. Elastic sleeves and bellows arrangements inherently require substantial length and also are limited in the number of parameters which can be adjusted to account for different conditions. Load carrying capability, misalignment capability, torsional deflection, fatigue resistance, economic constraints and other factors all vary with the circumstances, but in any event, the product should be easy to manufacture and easy to modify.
A precision flexible coupling can no longer be thought of as just a component to connect two misaligned shafts. It must be as precise in its performance as the servo motors, stepping motors, precision ground ball screws, rotary transducers, encoders and other like components used in the designs of today's machinery. The coupling must often have a high degree of torsional rigidity so that the components it is connecting will be as closely synchronized as possible. In elastic sleeves and bellows arrangements, however, torsional rigidity decreases and torsional deflection (wind-up) increases as a result of increasing misalignment. Thus, the need exists for an elastic coupler that can accommodate parallel and angular misalignment without a corresponding decrease in the torsional rigidity of the coupler.